Anisotropic Screening of Excitons in van der Waals Materials

Van der Waals (vdW) materials exhibit optical anisotropy due to the difference between in-plane (IP) covalent bonds and out-of-plane (OP) vdW bonds. In particular, InSe provides an ideal platform for exploring the optical characteristics of anisotropic excitons, as bandgap transitions involve both IP and OP dipoles. From polarization-resolved absorption spectra, we determined that the energy difference between excitons with IP and OP dipole responses in InSe is 0.4 meV. Based on many-body perturbation theory, we reveal that these excitons originate from different composite electronic states. Under photoexcitation with femtosecond pulses, the photocarriers screen the excitons and increase the energy difference between IP and OP optical responses to as much as 8 meV. This energy difference arises from the distinct screening lengths in the IP and OP directions and remains for over 100 ps at 77 K, while recovering faster, within 100 ps, at room temperature. We also examined the impact of carrier density on the anisotropic screening of excitons. Since this anisotropic exciton screening is linked to optical birefringence, this effect provides a foundation for researchers investigating vdW and other 2D materials.